As is well known in the art, the members of such a master-slave pair normally operate in synchronism though only the master communicates with associated peripheral units or other external components. Upon the occurrence of a failure in one of these members, the latter is dissociated from its twin which thereupon takes over the role of master if not already so designated.
In commonly owned U.S. application Ser. No. 347,315, filed Feb. 9, 1982 by Maria Grazia Corti et al as a continuation-in-part of an earlier application now abandoned, Ser. No. 293,997 filed Aug. 18, 1981 now abandoned, there has been disclosed a telecommunication system in which two special-purpose processors, interchangeably operating as master and slave, dialogue with a support processor designed to perform auxiliary functions such as call charging and traffic monitoring. The support processor intervenes in the operation of the special-purpose processors, as by checking for the absence of malfunctions and causing a master/slave changeover if the need therefor should arise; thus, the support processor may be regarded as being of higher rank than the two processors controlled thereby.
In a telecommunication system of the kind to which the invention is applicable, a pair of alternatively activable central processing units or CPUs mated in this manner are the principal constituents of each of several switching modules forming part of a digital telephone exchange or the like. These CPUs are under the control of a pair of support processors, common to all the modules, also operating in an interchangeable master-slave relationship. Thus, a malfunction of the support processor designated as the master will result in its automatic replacement by its formerly inactive twin. Since, however, either support processor must be able to communicate with either CPU of each switching module associated therewith, a four-way interconnection is needed between the two controlling processors and the two controlled CPUs of any module. Otherwise, with one support processor controlling only one CPU of each pair, a breakdown of the master CPU of a single module would require a master/slave changeover of the two support processors and of the CPUs of each module so that a subsequent breakdown of another CPU now acting as a master could no longer be rectified without manual intervention of an operator.
The aforementioned four-way interconnection can be realized with simple cables only if the switching modules are located close to the controlling support processors. In the case of longer lines it is necessary, for reliable operation, to insert a pair of mutually complementary modems at opposite ends of each line. This makes the network of connecting lines not only complex but also rather costly.